Process for producing iron-molybdenum alloy powder metal

ABSTRACT

PROCESS FOR PRODUCING IRON-MOLYBDENUM ALLOY POWDER METAL FROM MOLYBDENUM TRIOXIDE AND IRON OXIDE. THE OXIDES ARE BALLED AND HEAT TREATED IN A FURNACE IN THE PRESENCE OF A SOLID REDUCTANT. THE OXIDES ARE REDUCED TO METALLIC ELEMENTS, THE MOLYBDENUM DIFFUSED INTO THE IRON AND THE BALLS HARDENED TO FORM PELLETS. THE PELLETS ARE CRUSHED AND COMMINUTED TO A DESIRED SIZE.

United States Patent 3 737 301 PROCESS FOR PRODIICING IRON-MOLYBDENUM ALLOY POWDER METAL Vincent Hao Kwong Chu, Bethlehem, Pa., assignor to Bethlehem Steel Corporation No Drawing. Filed Dec. 30, 1971, Ser. No. 214,395 Int. Cl. B221? 9/00 U.S. Cl. 75.5 BA 15 Claims ABSTRACT OF THE DISCLOSURE Process for producing iron-molybdenum alloy powder metal from molybdenum trioxide and iron oxide. The oxides are balled and heat treated in a furnace in the presence of a solid reductant. The oxides are reduced to metallic elements, the molybdenum diffused into the iron and the balls hardened to form pellets. The pellets are crushed and comminuted to a desired size.

BACKGROUND OF THE INVENTION The invention is directed to a process for producing an iron-molybdenum alloy powder which can be processed by powder metal techniques into powder metal parts.

Powder metal parts made from metal powders containing iron and alloying elements do not have mechanical properties as good as parts made from conventionally prepared steel. It has been suggested that iron be alloyed with the appropriate element or elements in powder form. It is postulated that parts made from alloyed iron powder would have the same mechanical properties as parts made from conventionally prepared steels.

Attempts to produce such alloyed iron powders are exemplified by U.S. Pat. No. 3,424,572 issued Jan. 23, 1969 to Niranjan M. Parikh and titled Alloyed Metallic Powder Process. The invention is directed to mixing oxidic compounds of metals such as manganese, molybdenum, and nickel to form a mix of powder metals. The mix is heated to cause diffusion of the oxidic compounds to create a uniform product containing oxidic particles of the metals. The oxidic powders are then reduced to produce metallic powder. While the process does produce some alloyed powder the amount of alloying elements is small and the process is lengthy and expensive.

Another prior art practice is examplified by U.S. Pat. No. 2,799,570 issued July 16, 1957 to William A. Reed et al. Compounds of iron and other metals are mixed together and co-reduced by the use of hydrogen in the presence of hydrogen chloride gas. The chloride gas must be present in concentrates of 1% to 8% volume.

The above practices have found limited use in the field of powder metallurgy since it is inefiicient to diffuse iron and molybdenum as oxides and the use of HCl to rereduce iron oxides and other oxides is time-consuming and expensive.

It is an object of this invention to provide a process for producing an iron-molybdenum alloy powder in which oxides of iron and molybdenum are co-reduced to metallic iron and molybdenum and molybdenum is difi'used into the iron.

SUMMARY OF THE INVENTION Broadly, the invention includes forming a mix of a binder and oxides of molybdenum and iron, balling the mix, heat treating the balled material at a temperature for a time to reduce the oxides to metallic molybdenum and iron and diffuse the molybdenum into the iron and form pellets of iron-molybdenum alloy. The pellets are comminuted to final powder size.

DETAILED DESCRIPTION OF THE INVENTION A powder metal alloy of iron and molybdeum suitable for processing into powder metal parts can be made from the oxides of the metals. The oxides of the metals are mixed, balled, heated in the presence of a solid reductant and finally comminuted to a desired powder metal particle size.

Particles of molybdenum trioxide powder and a binder, for example, lignin and the like, are mixed with water to form a slurry. The molybdenum trioxide powder can have a particle size of -100 mesh sieve size or finer. The lignin binder can contain about 50% solids. The slurry thus formed can contain about 13.2% molybdenum trioxide, 42.5% lignin and 44.3% water. The slurry when mixed with the appropriate amount of iron oxide powder will produce an iron-molybdenum alloy containing about 1.0% molybdenum. If higher molybdenum contents, for example, up to 3.5 are desired in the alloy powder, the amount of molybdenum trioxide, lignin and water are altered to about 34.9% molybdenum trioxide, 31.8% lignin and 33.3% water.

In any case, the slurry is agitated for a time to obtain a uniform mixture. If desired carbon in the form of fine particles of graphite can be added to the slurry. The fine graphite particles should have a size such that 85% of the particles will pass a 100 mesh sieve. The fine graphite can be as much as 2.5% of the molybdenum trioxide in the slurry.

Iron oxide powder is added to the slurry. All the particles of the iron oxide powder passed a 100 mesh sieve size and 25% of the particles passed a 325 mesh sieve size. To produce an iron-molybdenum alloy powder containing 1.0% molybdenum, the mixture will contain about 1.2% molybdenum trioxide, 89.8% magnetite, 4.4% lignin and about 4.6% water. To produce an iron-molybdenum alloy powder containing 3.5% molybdenum, the mix should contain 4.7% molybdenum trioxide, 86.2% magnetite, 4.4% lignin and 4.5% water. As noted previously, the mix can contain about 0.25% carbon in the form of fine graphite.

After throughly mixing the above materials, the mixture is formed into green balls ranging in size from about of an inch to about of an inch, on a balling disc, drum, cone and the like. The green balls are dried by heating them in a furnace to a temperature range of about 200 F. to about 250 F.

The dried balls and a solid reductant, for example, coke breeze, coal, graphite and the like, are charged into one end of a furnace, for example, a rotary kiln and the like. The amount of solid reductant charged to the furnace is about to about of the weight of the dried balls. It is preferred to use graphite as the solid reductant in the form of a mixture of coarse graphite, having a particle size of mesh sieve size, and fine graphite having a particle size of 100 mesh sieve size. The ratio of coarse graphite to fine graphite can be between about 2 to 1 to about 4 to 1.

The charged materials are heated to within a temperature range of about 1000 F. to about 1250 F. in the presence of a carbon monoxide-rich atmosphere in a first zone in the furnace for a time sufficient to substantially completely reduce the molybdenum trioxide. A portion of the molybdenum trioxide is completely reduced to elemental molybdenum. The remaining molybdenum trioxide is reduced to molybdenum dioxide. At any rate, substantially all the molybdenum trioxide is reduced in the first heating step. A portion of the iron oxide can also be reduced to elemental iron in the first heating step.

As the dried balls and solid reductant continue to pass through the furnace, they become heated to within a temperature range of about 2000 F. to about 2100 F. in a non-oxidizing atmosphere in a second zone of the furnace. The charged materials are heated for a time to substantially completely reduce the molybdenum dioxide to elemental molybdenum and iron oxide to elemental iron. The elemental molybdenum is sufficiently small that it can be diffusedinto the crystal lattice of the elemental iron. It has been found that up to about 3.5 molybdenum can be diffused into the iron during the second heating step. As is well'known in the pelletizing art, heating the balls to within a temperature range. of 2000 F. to

2100 F. results'in producing hardened balls generally referred to as pellets. The pellets and'the residue of the solid reductant not used in reduction steps are discharged from the furnace into a chamber and are cooled'to a temperature below about 300 F.in a reducing or inert atmosphere. The pellets are separated from the solid re' ductant residue by screening.

The pellets are processed into a powder having particles substantially all of which will pass a 100 mesh sieve size. The pellets are comminuted byimpact grinding, although other forms of grinding, such as a ball mill, can be utilized. The powder is concentrated in dry magnetic-conoentrators wherein gangue material which might be present is separated from the iron-molybdenum alloy powder. Since the particles of the alloy powder are cold worked, that is, hardened by impact grinding, the particles are annealed at about 1700 F. to about 1900 F. in a hydrogen-rich atmosphere to soften the particles and to prevent reoxidation. The alloy powder is again passed through a dry magnetic concentrator for a final concentration. Ironmolybdenum powder produced by the above method can contain up to about 3.5 molybdenum. Of course, carbon can be added to the powder so that in further processing thereof, a steel-like material containing iron carbides and molybdenum carbides will be produced. 1

It will be understood that wherever percentages "are mentioned in the specification and claims, such percentages are in dry weight unless otherwise noted.

In a specific example of the invention, a slurry consisting of 13.2% molybdenum trioxide, 42.5% lignin and 44.3% water was agitated to obtain a uniform blend. A

high-purity magnetite concentrate containing about 99% iron oxide and a maximum of 0.4% HCl acid insoluble material and having a '100 mesh sieve was added to the slurry to form a mix consisting of about 1.2% molybdenum trioxide, 89.8% magnetite, 4.4% lignin and 4.6%

water. Sufiicient water was added to raise the moisture content to about 8% which is sufiicient for balling. The mixture was formed on a balling cone into balls having a diameter in the range of about V2 of an inch to-about A; of an inch. The balls were charged into a'batch electric furnace and heated to about 220 F. to be dried.

About 50 pounds of the balls were mixed with 40 pounds of graphite having a ratio of 3 parts coarse graphite with a particle size of +100 mesh sieve and 1 part of 100 mesh range of 2000 F. to 2100 F. and a non-oxidizing atmosphere. The balls and solid reductant are heated in this zone for about two hours. The pellets and thefresiduef'of the solid reductant were discharged from the-furnace into a chamber having a nitrogen rich atmosphere. The dis- (1) in a first zone in the kiln to a temperature for charged materials were cooled to about 250 F. The mate- Analysis of the pellets showed that the molybdenum was 1.1% and was diffused into the iron.

The pellets were comminuted in an impact grinder, dry magnet concentrated, annealed at a temperature of 185 0 F. in ahydrogen atmosphere and dry magnet concentrated. Substantially all the particles passed a mesh sieve size.

I claimi 1. Process for producing an iron-molybdenum alloy powder containing up to about 3.5% molybdenum comprising:

(a) forming a slurry of molybdenum trioxide powder,

a binder and water,

(b) adding iron oxide powder to the slurry to form a mixture containing oxides of molybdenum and iron,

* c)- forming the mixture into balls,

(d) drying the balls,

(e) charging the balls and a solid reductant into a furnace and heating the balls and solid reductant (l) to within a temperature range of about 1'000 F. to 0 F. in a carbon monoxide-rich atmosphere for a time to reduce a portion of the iron oxide to iron and to substantially completely reduce the molybdenum trioxide to molybdenum dioxide and molybdenum and (2) to within a temperature range of about 2000 F. to about 2100 F. in a non-oxidizing atmosphere for a time to substantially completely reduce molybdenum dioxide to molybdenum and iron oxide to iron, to diffuse the molybdenum I into the iron and to form pellets,

(f) discharging the pellets and solid reductant from the furnace and cooling the pellets and solid reductant to a temperature below 300 F. in an inert atmosphere,

(g) separating the pellets from the solid reductant,

(h) comminutingthe pellets to powder having a partic le size of '--100 mesh sieve,

(i)jconcentrating the powder by dry magnetic concentration,

(j) annealing the powder at 1850 F. in a hydrogen containing atmosphere, and

(k) concentrating the powder by dry magnetic concentration.

2. The process 'of claim 1 in which the binder of step (a) is lignin.

' '3. The process of claim 1 in which the solid reductant of step-(e) is graphite.

4. The process of claim 3 in which the graphite is in a ratio of 3 parts of coarse graphite having a particle size of +100 mesh sieve and 1 part of fine graphite having a particle size of 100 mesh sieve.

5. The process of claim 1 in which step (e)(l) is carried out in a first zone of said furnace and step (e) (2) is carried out in a second zone of said furnace.

6. The process of claim 5 in which the furnace of step (e) is a rotary kiln.

I 7. A process for producing an iron-molybdenum alloy powder having particles substantially all of which will pass a 100 mesh sieve size and containing up to about 3 .5. molybdenum, 1 comprising:

(a) forming a slurry of oxides of molybdenum and a binder and water, i

(b) adding oxides of iron to the slurry to form a mix containing oxides of molybdenum and oxides of iron, (c) forming the mix into balls, (d) drying the balls, (.e)* charging the balls and a solid reductant into one .end of a rotary kiln, and ".(f) heating the balls and solid reductant a time in a CO-rich atmosphere to partially reduce the oxides of molybdenum to elemental molybdenum and the oxides of iron to elemental iron, and

(2) in a second zone in the kiln to a temperature for a time in a non-oxidizing atmosphere to form pellets in which substantially all the oxides of molybdenum are reduced'to elemental molybdenum and substantially all the oxides of iron are reduced to elemental iron and to diffuse about 3.5% of the molybdenum into the iron,

(g) discharging the pellets and the solid reductant from the furnace,

(h) reducing the temperature of the pellets and solid reductant to below about 300 F. in an inert atmosphere,

(i) separating the pellets from the solid reductant,

(j) comminuting the pellets into a powder having a particle size of l mesh sieve size,

(k) concentrating the powder by dry magnetic concentration,

(l) treating the powder at a temperature for a time to soften the powder particles, and

(m) concentrating the powder by dry magnetic concentration.

8. The process of claim 7 in which the oxides of molybdenum step (a) are substantially completely molybdenum trioxide. 1

9. The process of claim 7 in which the binder of step (a) is lignin.

10. The process of claim 7 in which the oxides of iron of step (b) are substantially completely magnetite.

11. The process of claim 7 in which the solid reductant of step (e) is graphite.

12. The process of claim 11 in which the graphite consists of 3 parts of coarse graphite having a particle size of +100 mesh and 1 part of graphite having a particle size of 100 mesh.

13. The process of claim 7 in which the temperature of step (e)(1) is within a range of about 1000 F. to 125'0 F.

14. The process of claim 7 in which the temperature in step (e) (2) is within a range of about 2000 F. to about 2100 F.

15. A process for producing an iron-molybdenum alloy powder having a particle size of -100 mesh and containing up to about 3.5% molybdenum, said process comprising:

(a) forming a slurry of molybdenum trioxide powder,

lignin and water,

(b) adding magnetite powder to the slurry to form a mix therewith,

(c) forming the mix into balls,

(d) drying the balls in a furnace at a temperature within a range of about 200 F. to 250 F.,

(e) charging the balls and graphite into one end of a rotary kiln, the amount of graphite being from about to of the weight of the dried balls, said graphite being a mixture of 3 parts of coarse graphite having a particle size of mesh and 1 part of fine graphite having a particle size of 100 mesh,

(f) heating the balls and graphite in a first zone in the rotary kiln to within a temperature range of about 1000 F. to about 1250" F. for a time in a CO- atmosphere to partially reduce the molybdenum trioxide to molybdenum dioxide and partially to elemental molybdenum and to partially reduce the magnetite to elemental iron,

(g) passing the balls and graphite to a second zone in the rotary kiln,

(h) heating the balls and graphite in said second zone to within a temperature range of between about 2000 F. and 2100 F. in a non-oxidizing atmosphere for a time to substantially completely reduce the molybdenum dioxide to elemental molybdenum and the iron oxide to elemental iron and to diffuse up to about 3.5% elemental molybdenum into the elemental iron and to harden the balls into pellets,

(i) discharging the pellets and graphite from the rotary kiln,

(j) cooling the pellets and graphite to below about 300 F. in an inert atmosphere,

(k) separating the pellets from the graphite,

(l) comminuting the pellets to a powder having a particle size of -100 mesh,

(m) concentrating the powder by dry magnet concentration,

(n) heating the powder to a temperature range of about 1700 F. to about 1900 F. for a time in a hydrogen atmosphere to soften the powder,

(0) concentrating the powder by dry magnet concen tration.

References Cited WAYLAND W. STALLARD, Primary Examiner UNITED STATES PATENT owner QERTWEQATE @F @QRRhQTWN Patent No. 3,737,301 Dat d June 5, 1973 Inventor s) CHU VINCENT HAO KWONG It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 54, "re-reduce should read co-reduce Signed and sealed this 25th day of December 1973.

(SEAL) Attest:

EDWARD M. FLETCHER,JR RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents FORM PC4050 (1069) USCOMM-DC 60376-P69 I I 9 U.S. GOVERNMENT PRINTING OFFICE: 969 0-356-334, 

